The present invention relates generally to ferrules, molds for forming ferrules and methods for fabricating ferrules and, more particularly, to a ferrule having first and second ferrule body portions that have different nominal widths and an associated mold and fabrication method.
Optical fibers are utilized for a variety of applications, including data transmission and the like. In order to interconnect the optical fibers, fiber optic connectors are mounted upon the end portions of the optical fibers, and pairs of the fiber optic connectors are thereafter mated. In order to provide an optical connection with the relatively low attenuation and the small return loss now demanded by many applications, fiber optic connectors are generally designed such that fiber-to-fiber contact is established between the optical fibers upon which the respective fiber optic connectors are mounted. For example, fiber-to-fiber contact is preferably established between each optical fiber of a first fiber optic cable upon which a first fiber optic connector is mounted and the respective optical fibers of a second fiber optic cable upon which a second fiber optic connector is mounted, once the first and second fiber optic connectors have been mated.
In order to establish fiber-to-fiber contact, the front surface of the ferrule of each fiber optic connector must be extremely smooth and planar and must only have minimal, if any, angular errors relative to the optical fiber bores defined by the ferrule. In other words, the front surface of the ferrule preferably defines a planar surface that extends perpendicular to the longitudinal axes of the optical fiber bores. By way of example, the front face of most ferrules must generally have an angular error of less than 0.2xc2x0 relative to the optical fiber bores to ensure that the optical fibers upon which the ferrule is mounted can be brought into dry physical contact with the optical fibers of another fiber optic connector.
Although ferrules are typically molded to within relatively specific tolerances, the front surfaces of the ferrule are generally unable to be molded to have a sufficiently smooth front surface and to have a sufficiently small angular error. As such, the front surface of the ferrule must typically be polished after the ferrule has been mounted upon the end portions of the optical fibers. Not only does the polishing serve to smooth the front surface and to reduce the angular error of the front surface of the ferrule to within acceptable limits, such as less than 0.2xc2x0, but the polishing also serves to ensure that the ends of the optical fibers are properly positioned relative to the front surface of the ferrule, such as by either being flush with the front surface of the ferrule or by protruding by a predetermined amount relative to the front surface of the ferrule.
The front face of the ferrule is generally polished to have a predetermined angular relationship, such as 90xc2x0, with respect to the longitudinal axes of the optical fiber bores defined by the ferrule. Since the longitudinal axes of the optical fiber bores are inaccessible for use as a reference point or datum during polishing operations, ferrules are typically designed to have some other reference point or datum that has a predefined positional or angular relationship with respect to the longitudinal axes of the optical fiber bore. As such, the polishing of the front face of the ferrule can be done with respect to the datum that is accessible in order to appropriately polish the front face of the ferrule relative to the longitudinal axes of the optical fiber bores. In this regard, multifiber ferules having a substantially rectangular shape in lateral cross-section, such as an MT ferrule, generally include a shoulder between the ferrule shaft and the enlarged rear portion that serves as the datum for polishing purposes.
In order to take advantage of the efficiencies introduced by the factory assembly of the connectors, it is increasingly desired to polish the front face of the ferrule after the ferrule has been preassembled into a connector. Thus, the datum preferably remains accessible even after the ferrule has been assembled into a connector. As such, the connector can be efficiently assembled and the front face of the ferrule can still be polished after mounting the connector upon end portions of a plurality of optical fibers in the field. Unfortunately, the ferrule shoulder that serves as the datum for rectangular multifiber ferules is generally inaccessible once the connector has been assembled, thereby limiting the preassembly of connectors having rectangular multi fiber ferrules.
A significant portion of the shaft of a rectangular multifiber ferrule is typically accessible even after preassembly of the connector. Thus, consideration has been given to using the exterior surfaces of the shaft portion of a rectangular multifiber ferrule as the datum for polishing purposes, much like the outer diameter of a cylindrical ferrule serves as the polishing datum. Unfortunately, the exterior dimensions of a rectangular multifiber ferrule, cannot generally be defined to within small enough tolerances, such as +/xe2x88x925 microns, in order to effectively serve as a datum. In this regard, the exterior surfaces of a rectangular multifiber ferrule cannot be formed to within the exacting tolerances required for a polishing datum due to limitations in the molding process.
Rectangular multifiber ferrules are generally formed by injecting a plastic material, such as a thermoplastic or thermoset material, into a mold cavity defined by a pair of mold halves or components that mate along a parting line. Although the mold components can be formed of different materials, the mold components are typically formed of steel, such as D2 steel or stainless steel. The mold cavity as well as the resulting rectangular multifiber ferrule have a nominal thickness and a nominal width. Typically, the thickness of the rectangular multifiber ferrule can be defined to be within a very tight tolerance, such as to within 50 microns, of the nominal thickness during the initial fabrication of the mold. In this regard, the mold components are generally formed by an Electrical Discharge Machine (EDM) that cuts a steel workpiece with a wire. While forming the mold components by cutting a steel workpiece with a wire is efficient, the resulting mold components cannot generally be defined within tight enough tolerances, such as +/xe2x88x925 microns, to form parts that can serve as a datum during subsequent polishing operations. Thus, the mold components are typically further processed by removing mold material at the parting line of the mold until the thickness of the mold cavity is equal to the desired nominal thickness. For example, mold material is commonly removed from the parting line by a grinding process that is quite accurate.
Unfortunately, the width of a rectangular multifiber ferrule cannot be defined as precisely as the thickness according to conventional fabrication techniques. In this regard, inaccuracies in the width of a rectangular multifiber ferrule generally stem from two different causes. First, the width of the mold cavity as formed by the first and second mold components is not typically defined as precisely as the thickness of the mold cavity. In this regard, even though the mold can be formed such that the mold cavity is slightly undersized, it is relatively difficult to remove mold material from within the portions of the mold cavity defined by the first and second mold components so as to broaden the mold cavity until the actual width of the mold cavity approaches the desired nominal width.
In addition, the second cause for inaccuracies in the width of a rectangular multifiber ferrule stems from offsets that may occur in mating the pair of mold components along the parting line in order to define the mold cavity. In order to reduce the offset between the mold components, the mold is generally keyed such that a pin protruding from one mold component engages a hole defined by the other mold component. Even with the key, however, the mold components can sometimes be slightly offset in a widthwise direction. Since each mold component forms a respective portion of the resulting rectangular multifiber ferrule that has the same nominal width and has the same tolerance as the mold component, any offset between the mold components in a widthwise direction will cause a corresponding offset in the widthwise direction between the respective portions of the rectangular multifiber ferrule, thereby adversely affecting the precision with which the exterior surfaces of a rectangular multifiber ferrule can be defined. As a result of the cumulative effects of the difficulty in precisely defining the nominal width of a mold cavity and the inaccuracies that arise as a result of an offset between the mold components, the exterior surface of a rectangular multifiber ferrule cannot generally be defined with sufficient precision to serve as a datum during polishing operations.
While the shoulder of a rectangular multifiber ferrule serves as an effective datum for polishing purposes, it would be desirable for a rectangular multifiber ferrule to have a datum that is accessible even after the ferrule has been assembled into a connector. As such, the connector could be preassembled in the factory, such as by means of an automated process, and the connector could then be mounted upon the end portions of a plurality of optical fibers and a front face of the ferrule could be polished with respect to the datum in the field. Since the exterior surfaces of a rectangular multifiber ferrule are accessible even after the ferrule has been assembled into a connector, it would be desirable for a rectangular multifiber ferrule to define the exterior surfaces with sufficient precision that the exterior surfaces could serve as the datum for subsequent polishing operations. To date, however, the exterior surfaces of a rectangular multifiber ferrule have not been consistently defined with sufficient precision to effectively serve as a datum during polishing operations.
According to the present invention, a ferrule is therefore provided that has an exterior surface that can be defined with sufficient precision to serve as a datum during subsequent polishing operations. A mold for forming the ferrule and a method of fabricating the ferrule are also provided according to other aspects of the present invention. Since the exterior surfaces of the resulting ferrule can serve as the datum during such polishing operations, the ferrule can be assembled into a connector prior to mounting the ferrule upon the end portions of a plurality of optical fibers, thereby permitting the ferrule to be preassembled in a factory setting, such as by an automated process.
The ferrule includes first and second ferrule body portions that are joined along a parting line. At least one of the ferrule body portions defines at least one optical fiber bore extending lengthwise through the ferrule. More typically, the first and second ferrule body portions cooperate to define a plurality of optical fiber bores extending lengthwise through the ferrule in order to define a multifiber ferrule.
According to the present invention, the first ferrule body portion has a first width and the second ferrule body portion has a second width that is less than the first width by at least 50 microns. As a result of the difference in the widths of the first and second ferrule body portions, the resulting ferrule typically has a ledge extending lengthwise along the parting line. In addition, the width of the first ferrule body portion is defined to be within a first tolerance, while the width of the second ferrule body is defined to be within a second tolerance that is larger than the first tolerance. For example, the second tolerance of the second ferrule body portion can be at least two times larger than the first tolerance of the first ferrule body portion. As such, the first ferrule body portion is not only larger than the second ferrule body portion, but the first ferrule body portion is also more precisely defined. Thus, only one portion of the ferrule of the present invention needs to be precisely defined, thereby simplifying the fabrication process.
According to the present invention, the first and second ferrule body portions are capable of being offset in a widthwise direction by up to a maximum offset. As such, the first width of the first ferrule body portion is preferably larger than the second width of the second ferrule body portion by at least the sum of the first and second tolerances and two times the maximum offset between the first and second ferrule body portions. As a result of the reduced width of the second ferrule body portion relative to the first ferrule body portion, any offset between the first and second ferrule body portions in a widthwise direction up to the maximum offset will not cause the second ferrule body portion to protrude outwardly beyond the first ferrule body portion in the widthwise direction. As such, the exterior surfaces of the first ferrule body portion can continue to serve as a datum during subsequent polishing operations even though the first and second ferrule body portions may be offset.
By reducing the width of one of the ferrule body portions relative to the other ferrule body portion and by only requiring the larger of the ferrule body portions to be fabricated to an exact tolerance, the ferrule of the present invention is not as susceptible to the two primary causes of inaccuracy in its exterior dimensions. In addition, the precision with which the exterior surfaces of the first ferrule body portion can be defined is not hindered by offsets between the first and second ferrule body portions such that the exterior side surfaces of the ferrule can continue to serve as a datum during polishing operations. In this regard, the ferrule can be more efficiently fabricated since only one of the ferrule body portions needs to be precisely defined in a widthwise direction in order to provide a precision exterior surface that serves as a datum in subsequent polishing operations.
According to another aspect of the present invention, a mold is provided for forming a ferrule, such as a multifiber ferrule. The mold includes first and second mold components that mate along the parting line and that cooperate to define a mold cavity within which the ferrule is formed. The first mold component defines the width of one portion of the mold cavity to within a first tolerance of a first nominal width. Likewise, the second mold component defines the width of another portion of the mold cavity to within a second tolerance of a second nominal width. Notably, the second nominal width of the second mold component is smaller than the first nominal width of the first mold component. For example, the second nominal width of the second mold component can be smaller than the first nominal width of the first mold component by at least 50 microns. In addition, the second tolerance is larger than the first tolerance. For example, the second tolerance of the second mold component can be at least two times larger than the first tolerance of the first mold component. As such, the portion of the ferrule formed by the first mold component is not only wider, but is also defined to within more exacting tolerances than the portion of the ferrule that is concurrently formed by the second mold component. Accordingly, the mold of the present can be more efficiently fabricated since only one of the mold components, i.e., the first mold component, needs to be formed to have a width that is defined to within the more exacting tolerances required to serve as a datum, such as +/xe2x88x925 microns.
In forming the ferrule, the first and second mold components can be offset in a widthwise direction by up to a maximum offset. According to one advantageous embodiment of the present invention, however, the first nominal width of the first mold component is larger than the second nominal width of the second mold component by at least the sum of the first and second tolerances and two times the maximum offset. As such, the first and second mold components can be offset by up to the maximum offset in the widthwise direction while still permitting the exterior surfaces of that portion of the ferrule formed by the first mold component such that the exterior surfaces of that portion of the ferrule, i.e., the first ferrule body portion, can effectively serve as a datum during subsequent polishing operations.